I throw in kale, or other greens, sometimes.
My favorite lentils are dark green french ones known as "puy"
Tomato, or tomato paste, adds depth. I don't like to use chicken broth in lentil soup - I'd rather start with water and build up without the meat.
(Although of course I make the soup with bits of lamb or merguez sausage sometimes.)
If I have a parmasean rind, that goes in . .
Shallots, garlic, lots of salt and pepper . . . if I have them, carrots, celery, roasted red pepper
Spice variations involve cumin, harissa or sirarcha hot sauce.
Not purely a lentil soup, but one of my favorite soups is the Lentil Minestrone from Vegetarian Cooking for everyone. Use the Mushroom Soy Sauce to boost flavor if you can get it in your area, but it is a very delicious soup. I make it with chard but you can use spinach. This has a 'minestrone' background flavor profile.
Greens do go very well with lentils. I recently made a very simple and quick curried lentil and spinach soup recipe from Gourmet's yellow book that I liked. The squeeze of lemon at the end is a great fresh touch.
Another Armenian thing from my ex-wife's grandma is chard and lentil soup. Not really different from any decent lentil soup, except that after the lentils have reached the edible stage you strip the leaves from the stalks of a bunch of chard, chop the stalks and add them. When they're reasonably tender you chop and add the leaves, and stir them in (you could use beet greens too, or even spinach). When it's all done, adjust the seasoning with fresh-squoze lemon juice. I've made this both meatless and with sausage chunks - both nice.